The present invention relates generally to a sheet flattening method and apparatus for flattening of a paper sheet which has been printed with a liquid ink. More particularly, the sheet flattener is configured for use as a portion of a printing system, such as a duplicator, which may also include unprinted sheet storage bins, an ink jet printer, drying apparatus, and the like, along with means for transporting the sheets through the duplicator.
Certain duplicators utilize a water-base ink for printing sheets of paper. One example of such a duplicator is shown in co-pending U.S. application Ser. No. 355,799, filed Mar. 8, 1982, now U.S. Pat. No. 4,413,265 in which an ink jet printer is used. The liquid ink is deposited upon the paper sheet in droplets, which are subsequently dried as the sheet is moved through an appropriate drying apparatus.
On average, a sheet is printed in a duplicator with approximately 9-10% of the sheet area being covered by the printing ink. In instances where the proportion of printed area to sheet area is significantly higher than normal, the sheet tends to curl toward the printed side of the sheet as the ink dries. A similar situation can exist where smaller areas on the sheet are particularly heavily printed. The radius of curvature produced by this curl may be as severe as 1" (2.5 cm).
Such curling of heavily printed sheets detracts from the appearance and interferes with the stacking of completed copies. More importantly, after printing, these sheets are carried by transport apparatus through the remainder of the printing system and/or any subsequent finishing apparatus. The curling can result in portions of the sheets, and particularly their leading edges, being carried outside of the normal transport path. This in turn can lead to the sheets being inadvertently deflected from the transport path, jamming the transport apparatus or damaging the sheets.
What is needed, therefore, is an apparatus and method which may be incorporated into a duplicator or other printing system to flatten and recondition heavily printed sheets which have become curled by the printing process.